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- """Encode valid C string literals from Python strings.
- If a character is not allowed in C string literals, it is either emitted
- as a simple escape sequence (e.g. '\\n'), or an octal escape sequence
- with exactly three digits ('\\oXXX'). Question marks are escaped to
- prevent trigraphs in the string literal from being interpreted. Note
- that '\\?' is an invalid escape sequence in Python.
- Consider the string literal "AB\\xCDEF". As one would expect, Python
- parses it as ['A', 'B', 0xCD, 'E', 'F']. However, the C standard
- specifies that all hexadecimal digits immediately following '\\x' will
- be interpreted as part of the escape sequence. Therefore, it is
- unexpectedly parsed as ['A', 'B', 0xCDEF].
- Emitting ("AB\\xCD" "EF") would avoid this behaviour. However, we opt
- for simplicity and use octal escape sequences instead. They do not
- suffer from the same issue as they are defined to parse at most three
- octal digits.
- """
- from __future__ import annotations
- import string
- from typing import Final
- CHAR_MAP: Final = [f"\\{i:03o}" for i in range(256)]
- # It is safe to use string.printable as it always uses the C locale.
- for c in string.printable:
- CHAR_MAP[ord(c)] = c
- # These assignments must come last because we prioritize simple escape
- # sequences over any other representation.
- for c in ("'", '"', "\\", "a", "b", "f", "n", "r", "t", "v"):
- escaped = f"\\{c}"
- decoded = escaped.encode("ascii").decode("unicode_escape")
- CHAR_MAP[ord(decoded)] = escaped
- # This escape sequence is invalid in Python.
- CHAR_MAP[ord("?")] = r"\?"
- def encode_bytes_as_c_string(b: bytes) -> str:
- """Produce contents of a C string literal for a byte string, without quotes."""
- escaped = "".join([CHAR_MAP[i] for i in b])
- return escaped
- def c_string_initializer(value: bytes) -> str:
- """Create initializer for a C char[]/ char * variable from a string.
- For example, if value if b'foo', the result would be '"foo"'.
- """
- return '"' + encode_bytes_as_c_string(value) + '"'
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